This study proposes new techniques for overcoming the two major obstacles to practical protein fractionation by ultrafiltration: (1) the diffuse pore-size distribution of the membrane, which allows only gross separation and (2) the buildup of retained proteins on the membrane surface, drastically decreasing selectivity and separation rate. Membranes having sharp size cut-off are formed by crosslinking proteins of appropriate size into the membrane pores they can penetrate. Then molecules larger than this protein can no longer permeate. Use of a pair of such membranes with different cut-offs would isolate a fraction of narrow size range. Pilot studies are described on blocking of pores with gamma-globulin. The technique could also be applied to chromatographic gels. The surface protein layer which builds up during ultrafiltration is kept at negligible thickness by pulsed electrophoresis, which frequently moves these proteins up into the stirred bulk solution. The combination of the two techniques would be of considerable help in research on blood biochemistry and could aid in large-scale fractionation of plasma for component therapy.